The present invention relates to a pressure detecting apparatus that converts the pressure detected thereby to an electrical signal and outputs the converted electrical signal. Specifically, the present invention relates also to a pressure detecting apparatus that exhibits excellent thermal response.
Usually, the semiconductor pressure sensor chip that employs the so-called piezoresistance effects has been used for a pressure detecting apparatus for measuring the intake air pressure on the air intake side of an engine in the electronic controlled fuel injection apparatus for automobiles. Since the operational principles of the pressure detecting apparatus that employs the semiconductor pressure sensor chip as described above are well known, the detailed descriptions thereof are omitted. The pressure detecting apparatus includes a bridge circuit consisting of semiconductor strain gauges formed on a diaphragm made of a material that exhibits piezoresistance effects such as single crystalline silicon. A pressure is detected by taking out the gauge resistance changes, caused in the semiconductor strain gauges by the diaphragm distortion, from the bridge circuit in the form of an electrical signal.
Now the pressure detecting apparatus briefly described above will be explained below with reference to FIGS. 5 and 6. FIG. 5 is a cross-sectional view of a conventional pressure detecting apparatus. FIG. 6 is an expanded cross-sectional view showing a part of the conventional pressure detecting apparatus shown in FIG. 5. Referring now to these drawings, a pressure detecting apparatus 500 includes a pressure detecting device 501, that is a semiconductor pressure sensor chip, mounted on a housing base 502 of a resin molding, that is a package casing of pressure detecting apparatus 500. A housing recess 503 for housing pressure detecting device 501 therein is formed in housing base 502.
Pressure detecting device 501 is mounted on housing base 502 in such a configuration, in which pressure detecting device 501 is bonded by die-bonding with an adhesive 504 to housing recess 503 formed in housing base 502. Pressure detecting device 501 is electrically connected, via bonding wires 506, to lead terminals (lead frames) 505 integrated into housing base 502 by insertion molding such that lead terminals 505 are extending through housing base 502.
For reducing the stress exerted from housing base 502 in the structure described above, pressure detecting device 501 is bonded to a pedestal 507 made of glass by the anodic bonding technique known to those skilled in the art such that a vacuum reference space is formed between pressure detecting device 501 and glass pedestal 507. A gel protecting material 508 covers the surface 501a of pressure detecting device 501 and adheres pressure detecting device 501 to housing base 502 in such a manner that gel protecting material 508 contains bonding wires 506 therein. Protecting material 508 protects pressure detecting device 501 from the contaminants contained in the not-shown medium, the pressure thereof is to be measured with pressure detecting apparatus 500, and transmits the medium pressure to pressure detecting device 501. Protecting material 508 is also disposed between the side face of detecting device 501 and the side face of housing recess 503.
A housing cover 510 formed of a molded resin material includes a tube-shaped pressure transmitting section 509 having a cylindrical inner surface 509a (cf. FIG. 5). Housing cover 510 is mounted on and fixed, with an adhesive, to the opening side end portion of housing recess 503 in housing base 502 such that a pressure detecting space 511 consisting of a space connected to pressure transmitting section 509 is formed (cf. FIG. 5). The medium pressure to be measured is transmitted to pressure detecting space 511 through pressure transmitting section 509 in housing cover 510. Pressure detecting apparatus 500 detects the pressure difference between the transmitted medium pressure to be measured and the vacuum reference room pressure as a pressure change, converts the detected pressure change to an electrical signal in pressure detecting device 501, and outputs the converted electrical signal. Thus, the absolute medium pressure is measured.
For meeting the various demands for pressure detecting apparatus 500 such as down-sizing of entire pressure detecting apparatus 500, realization of very precise detection characteristics and realization of very high reliability, the opening size of housing recess 503 is optimized so that a clearance optimum for reducing the stress exerted from housing base 502 may be obtained between pressure detecting device 501 and housing base 502 (cf. Japanese Patent Publication No. 2003-247903).
In pressure detecting apparatus 500 having the structure as described above, the deformation of housing base 502 caused by an external stress exerted from housing cover 510 or by a thermal stress due to a severe measurement environment associating drastic temperature changes adversely affects the detection performances of pressure detecting device 501, impairing the thermal response of pressure detecting apparatus 500.
The thermal response is one of the evaluation items for performances tests indicating the detection performances change caused by the environmental temperature change, e.g. from a high temperature to a low temperature. In the pressure detecting apparatus, the thermal response thereof is not good, variations are caused between the initial detection performances and the detection performances after a temperature change is caused.
If the loading amount of adhesive 504 for mounting pressure detecting device 501 on housing base 502 is too large, adhesive 504, which has bulged out of the gap between the bottom surface 503a of housing recess 503 and the bottom surface 507a of pedestal 507 creeps up the clearance between pressure detecting device 501 and housing base 502, that is, the gap between the side face 507b of pedestal 507 and the side face 503b of housing recess 503 as shown in FIG. 6. Therefore, the stress caused, for example, by the deformation of housing base 502 in the direction indicated by the outline arrows in FIG. 6 directly affects the detection performances of pressure detecting device 501, impairing the thermal response of pressure detecting apparatus 500.
In view of the foregoing, it would be desirable to provide a pressure detecting apparatus that facilitates reducing the adverse effects of thermal stress on the detection performances to the extreme thereof and exhibits excellent thermal response.
Further objects and advantages of the invention will be apparent from the following description of the invention and the associated drawings.